INTER- AND INTRA-TESTER MEASUREMENT RELIABILITY OF THE RATIO OF ISOMETRIC INTERNAL TO EXTERNAL SHOULDER ROTATION STRENGTH USING A HAND HELD DYNAMOMETER IN HEALTHY VOLUNTEERS

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Abstract

Objective
To evaluate the intra- and inter-tester reliability of measuring shoulder
external rotation (ER) and internal rotation (IR) strength and the ratio
between the two (ER:IR) using a hand held dynamometer (HHD) and to
establish a reliable protocol for objective measurement. The study was
conducted on a cohort of healthy military and civilian participants between
18 to 40 years old.
Design
Cohort study
Setting
British Army Regional Rehabilitation Unit.
Participants
25 healthy volunteers (17 male, 8 female), aged 18 – 40 (mean ±
standard deviation) 30.9 ± 6.3 years, body mass = 80.6 ± 15.1 kg, and
stature = 1.73 ± 0.08 m. (17 males aged 33.1 ± 6.1 years, body mass =
87.3 ± 10.9 kg, and stature = 1.8 ± 0.1 m. Eight females aged 26.1 ± 4.0
years, body mass = 66.3 ± 13.0 kg, and stature = 1.7 ± 0.04 m).
Method
Shoulder ER and IR isometric muscle strength was measured on two
occasions seven days apart using the HHD. Subjects sitting with their
glenohumeral joint in neutral and elbow flexed to 90 degrees, conducted
three x six second tests for ER and IR. Two testers administered the test
to each subject within each session, separated by a 10 minute rest. The
level of agreement between strength tests was quantified using the
3
Bland-Altman plotting method and applying the 95% limits of agreement
(LoA). The 95% standard error of measurement (SEM) was calculated to
quantify variation in individual performance. As a comparator to previous
research, ICC (3,1) was also reported.
Results
ER measurements showed the best agreement with the 95% LoA of 0.3 ±
3.0 kg (Meandiff ± (1.96 × SDdiff)). The SEM was 2.1 kg and ICC = 0.961.
The IR measurements showed less agreement with the 95% LoA ranging
from 0.2 ± 6.1 kg, a greater SEM of 4.3 kg and ICC = 0.959. Ratio data
showed the least agreement between trials with the 95% LoA of 4.0 ±
20.3%, a large SEM of 14.4% and lower ICC = 0.745.
Conclusion
The study demonstrated that measuring the ratio is not a reliable method
but individual scores of ER more than IR are considered reliable. Caution
is to be taken when measuring strong participants, as more variation in
the scores is obtained. The methodology described can easily be used
as protocol in a clinical setting.

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